1. Technical Field
This disclosure relates to the fabrication of thin film magnetic read/write heads and particularly to the design of a DFH (Dynamic Flying Height) slider air bearing surface (ABS) to achieve high DFH efficiency, back-off efficiency and uniform touchdown detectability across a disk.
2. Description
The present disclosure relates to a hard disk drive (HDD). More specifically, the present disclosure relates to an air bearing surface (ABS) design for a slider that facilitates improving the uniformity of touch-down detection DHS efficiency and back-off efficiency across the stroke for achieving sub-nanometer active clearance and resistance to HDI events such as contact with disk surface lubricants.
In a HDD, the data on a disk is read and written by a magnetic transducer (or read/write head), and each such transducer is embedded within a slider which is mounted on a suspension and flies over the rotating disk with a passive spacing of around 10 nm. During the read/write process, the active spacing is actually reduced, perhaps to below 1 nm, in order to achieve a higher areal density and disk capacity (eg. 1 Tbpsi). The current process of achieving such low fly heights, which is the so-called fly on demand (FOD) or dynamic fly height (DFH) process, controls magnetic spacing via the local thermal protrusion produced by activation of a heater embedded near the transducer.
In order to control the active spacing accurately when applying power to the heater, it's necessary for the slider to find a stable touch-down point as a baseline and then be backed off to a desired spacing for the read/write operation. Typically the backing off process is implemented by application of the well-known (and, therefore, not described here) Wallace Equation algorithm, which has been widely employed in the HDD industry to calculate the relative spacing change in real time.
When heater protrusion causes the slider to touch the rotating disk, many interesting (and typically undesirable) dynamics phenomena appear, to which stiff (high air pressure on ABS) and soft (low air pressure on ABS) air-bearing sliders respond quite differently. At such low clearances, the contact dynamics becomes extremely critical. The dynamics will not only affect touch-down detection and further impact the accuracy of back-off spacing settings, but it will also influence HDD reliability through increased wear, instability and modulation. This is particularly problematic if the head vibrates vigorously after hitting a particle or droplet of lubricant during operation.
Recent experiments and simulations show that there are two stages in the touch-down process. The first stage is soft touch-down (STD, see FIG. 1), producing a low frequency vibration (say, 60 kHz˜120 kHz) which is a suspension mode excited by the interference of lubricant on the disk. The second stage is hard touch-down (HTD) with a high frequency vibration (say, 200 kHz˜400 kHz) which is the second pitch mode of an air bearing slider excited by the contact between head and disk. Usually a late HTD tends to cause wear due to severe head/disk contact. This occurrence should definitely be prevented in a HDD. On the other hand, to mitigate an early STD requires a soft (i.e., low air pressure) ABS. Unfortunately, such a low pressure ABS might fail to follow disk topography accurately and have a higher risk of fly height (FH) modulation when head/disk interference (HDI) happens during operation.
To ensure a successful detection of STD without the concern of FH modulation, the air bearing stiffness, especially the local stiffness on the heater bulge, needs to be well controlled in an optimal range. ABS stiffness has proven to be one of the dominant factors of touch-down detectability, and a parameter called “integrated force” (IGF) is introduced to quantify the local stiffness in an ABS design. The IGF is the integration of air pressure on the heater-caused protrusion when turning on the heater to achieve a desired active spacing. ABS design with too high IGF will skip the STD phase of a touch-down and cause wear in the HTD phase; while an ABS design with too low IGF will suffer FH modulation, although there is no concern about STD detection. As mentioned above, after detecting touch-down, the slider will be backed off to a desired active spacing for the subsequent read/write operation.
In addition to affecting touch-down detectability, IGF also governs the back-off efficiency. In FOD or DFH applications, when the heater is turned on, there will be a protrusion-induced increase in the air pressure acting on the slider due to the squeezed layer of air within the head/disk interface. The higher pressure is applied to the area of heater protrusion, thus the stiffest local portion of the air bearing layer is at the heater region. This additional air pressure will act counter to the change of fly height spacing, and bring on what is called push-back or fly height compensation. This is the counterproductive effect of actually preventing the local deformation produced by the heater, which is required to yield good DFH or FOD efficiency.
Basically the DFH efficiency is defined as the ratio of the fly height spacing change to the additional height produced by heater-caused protrusion (or, equivalently, to heater power). As a result, in ABS design, a well-controlled IGF will play an important role in achieving uniform touch-down detection and back-off efficiency across the entire disk surface for excellent resolution in read/write operations.
Various approaches have been suggested for achieving higher DFH efficiency. For example, isolated micro pads proposed by Zhang (US Publ. Pat. Appl. No. 2008/0117550 A1) and a bridged micro pad proposed by Shen et al. (U.S. Pat. No. 7,969,685 B2) were disclosed to reduce the push-back effect induced by heater protrusion by reducing the air pressure on the micro pad where read/writer element and heater embedded. While such approaches, along with those of Zhang et al. (U.S. Pat. No. 7,936,538 A1) and Bolsana et al. (US Publ. Pat. Appl. 2011/0026164 A1) attempt to improve DFH efficiency, they ignore relevant issues that will be addressed in the present application and generally fail to achieve comparable results.
It must also be noted that at such low clearances, a variety of proximity interactions, such as intermolecular forces (IMF), meniscus forces and electrostatic force (ESF), along with the increased influence of disk topography, all tend to destabilize the air-bearing slider. Moreover, operating shock, lubricant pickup and environments (altitude/thermal/humidity) sensitivities are also challenges to drive reliability.
Referring to schematic FIG. 1, there is shown a side view of a suspension (5) mounted DFH slider (10) undergoing a soft touchdown (STD) as a result of heater (35) protrusion (20) of the ABS region about the read/write elements (30) located at the trailing edge of the slider. The protruded trailing edge of the slider contacts a layer of lubricant (40) covering the surface of a rotating disk (50) causing the STD. It will be highly desirable to eliminate such events.
Without well-controlled ABS stiffness and IGF on the heater protrusion, the slider/disk contacts would lead to either HTD and wear during touch-down process or FH modulation and instability when HDI occurs. Therefore, there is a need in HDD industry to employ a new concept in ABS topographic features that can sufficiently aid in applications where sub-nanometer clearances are dominant. The present application provides a total solution for uniform touch down detection and back-off efficiency without compromising ABS stiffness and HDD performance.